This invention relates generally to biopolar transistors, and more particularly, to silicon pipolar transistors of the type having a tunnel barrier associated with the emitter region. There is a need for bipolar transistors with lower base resistance, both in power transistor applications and in high-frequency transistor applications. A low base resistance generally results in enhanced device performance. Although the base resistance can be easily reduced by increasing the impurity concentration in the base region of the transistor, this will also have the effect of reducing the common emitter current gain, and it is usually desirable to keep the common emitter current gain as high as possible. For a given fabrication technology, the ratio of common emitter current gain to base resistance is practically constant, which means that a lower base resistance can only be achieved at the expense of reduced common emitter current gain.
A barrier emitter transistor makes use of an insulating or semi-insulating barrier associated with the emitter region of the transistor. Majority and minority current carriers, i.e. electrons and holes, are transmitted through the barrier by means of a process described as quantum mechanical tunneling. By inhibiting minority carrier flow a tunneling barrier can be used to improve the injection efficiency, and hence the common emitter current gain, of a transistor. The injection efficiency is the ratio of the set majority-carrier current across the base-emitter junction, to the net minority-carrier current across the same junction. If the injection efficiency is increased, so is the common emitter current gain of the transistor. This increase provides the basis on which tunneling barriers have been used to enhance common emitter current gain of a transistor without significant increase in the base resistance. One group of researchers in this area has recognized that semi-insulating polycrystalline silicon (SIPOS) material may be used as a barrier layer, but it has not been conclusively established that SIPOS consists of tunneling barriers. See T. Matsushita et al., "A Silicon Heterojunction Transistor," Appl. Phys. Lett., Vol. 35, No. 7, October, 1979, pp. 549-50.
The use of emitter tunneling barriers has, however, raised another problem. The presence of the tunneling barrier significantly increases the emitter resistance of the transistor, and this is undesirable from a performance standpoint. Ideally, then, a bipolar transistor structure should provide low base resistance and high common emitter current gain, without significant increase in emitter resistance. In spite of improvements in emitter barrier structures, there is still a need for a transistor having these characteristics. The present invention fulfills this need.